Characterization of catastrophic flood-related features in the English Channel

Abstract

Megaflood flows are global-scale phenomena that can have a significant impact on Earth's landscapes and climate. The low frequency occurrence of these events prevents their direct observation. The analysis of ancient megaflood terrains and bedform associations are the primary resources for the understanding of catastrophic flows. Previous studies have proposed the palaeo-channel network present at the English Channel seafloor to have been carved by catastrophic flows. This thesis investigates the origin of the English Channel palaeovalley network, testing the hypothesis of its catastrophic formation and aiming to reconstruct the relative timing and magnitude of the events that formed the palaeo-channel system. Seismic reflection data reveal the presence of depressions up to 100 m-deep carved into bedrock at the proposed spill-point. New high-resolution bathymetric data show kilometre-scale channels and associated erosional bedforms that extends for more than 200 kilometres from the Dover Strait to the Central English Channel. The stratigraphic and geomorphologic analyses indicate the observed features are similar to other flood-generated features from other well-established megaflood terrains on Earth. The quantitative characterisation of the bedforms presented in this work, together with interpretation of cross-cutting relationships of the mapped erosional surfaces, allow for the reconstruction of the relative history of the catastrophic events that carved the palaeovalley network and led to the separation of Britain from Europe through the breaching of the rock ridge present at the Dover Strait. The detailed interpretation of the 100 m-deep bedrock depressions located at the proposed breach point allows for a reconstruction of the rapid erosion of the Dover Strait. Geomorphologic analyses of the palaeo-channel network and its associated bedforms and relation to bedrock geology are presented in this work thanks to available high-resolution bathymetry and seismic reflection data. Finally, the magnitude of the palaeo-flows, estimated through palaeo-hydraulic calculations, gives further evidence of the catastrophic nature of the flows. The proposed model and relative history of the English Channel megaflood is presented in agreement with palaeo-geographic and palaeontological studies previously carried out in the study area, contributing to the reconstruction of the Pleistocene palaeo-geography of north-western Europe.